Various improvement of Functor framework implementation

• drop backwards compatible code which allowed compilation with C++17; now require at least C++20
• constrain template arguments with concepts
• replace enable_if SFINAE tricks with requires
• drop the internal (and expensive to compile!) prepared wrapper which adds some overloads to the call operator, and instead use a lambda which forwards to a dedicated, overloaded invoke function which does the wrapping
• generalize Flatten to work with any range-of-ranges (not just vector-of-vector)
• streamline Filter functor implementation
• improve Tuplet implementation
• streamline FunctorArg parsing
• improve ParticlePropertyUser functor -- no longer needs to be template, instead use a variant to represent the two allowed representations of its state
• drop some redundant deref_if_ptr hacks
• prefer COUNT_IF(predicate) over SIZE_OF @ FILTER(predicate)

goes together with DaVinci!1190 (merged)